We are using mutants of the yeast, Saccharomyces cerevisiae, to gain an understanding of the cell division process in eucaryotic cells. These mutants have identified 32 genes whose products play specific and necessary functions for the completion of cell division. We have devised a new procedure for determining the normal sequence of action of two gene products. Using this protocol we have determined the sequence with which three gene products act to assure the initiation of DNA synthesis in yeast. Additional inhibitors are being sought that are cycle specific in order to continue this sequencing procedure further into the cell cycle. A number of observations suggest the expression of the genome is organized with respect to the cell cycle. We are examining this possibility by looking at the synthesis of specific proteins within the cell cycle. We are also examining the possibility that this program of gene expression is coordinated with events of the cell cycle by looking at the synthesis of proteins and mutants blocked at specific events. Since many of the mutants are blocked in functions that are specific to the nucleus of the cell, for example DNA synthesis and nuclear division, we will attempt to characterize nuclei isolated from various parts of the cell cycle and from mutants blocked at different steps of the cell cycle. The nuclei will be examined by disc gel electrophoresis for their protein composition and by electron microscopy for their ultra structure. It is our hope that we will be able to characterize a program of changes in protein composition for the nucleus as a whole and for subcomponents within the nucleus as a function of the cell cycle.